High free volume membrane for gas separation

ABSTRACT

A gas separation membrane, methods of forming the membrane, and methods of using the membrane for gas separation are provided. An exemplary gas separation membrane includes a cellulosic matrix and a polymer of intrinsic microporosity (PIM). The PIM includes chains coupled by a heat-treating under vacuum.

TECHNICAL FIELD

The present disclosure is directed to polymeric membranes for gasseparation. More specifically, the membranes are formed from blends ofglassy polymers with high free volume polymers.

BACKGROUND

Natural gas supplies 22% of the energy used worldwide, and makes upnearly a quarter of electricity generation. Further, natural gas is animportant feedstock for the petrochemicals industry. According to theInternational Energy Agency (IEA), the worldwide consumption of naturalgas is projected to increase from 120 trillion cubic feet (Tcf) in theyear 2012 to 203 Tcf by the year 2040.

Raw, or unprocessed, natural gas is formed primarily of methane (CH₄),however it may include significant amounts of other components,including acid gases (carbon dioxide (CO₂) and hydrogen sulfide (H₂S)),nitrogen, helium, water, mercaptans, and heavy hydrocarbons (C₃₊), amongother components. These contaminants must be removed during gasprocessing in order to meet the standard pipeline specifications ofsales gas. In particular, the removal of acid gases (CO₂ and H₂S) hasbeen a significant research topic due to the problematic effects of acidgases on natural gas heating value, pipeline transportability, andpipeline corrosion in the presence of water.

Currently, the majority of gas processing plants remove CO₂ and H₂S fromnatural gas by absorption technology, such as amine adsorption. However,several drawbacks are associated with this technology, including energyusage, capital cost, maintenance requirements, and the like.

SUMMARY

An embodiment described in examples herein provides a gas separationmembrane. The gas separation membrane includes a cellulosic matrix and apolymer of intrinsic microporosity (PIM). The PIM includes chainscoupled by a heat-treating under vacuum.

Another embodiment described in examples herein provides a method forforming a gas separation membrane. The method includes forming acellulosic polymer solution, forming a polymer of intrinsicmicroporosity (PIM) solution, and blending the polymer solution of thecellulosic polymer with the polymer solution of the PIM to form a mixedpolymer solution. A dense film is formed from the mixed polymersolution. The dense film is dried. The dense film is heat-treated underheat and vacuum to couple the PIM chains.

Another embodiment described in examples herein provides a method forremoving at least a portion of an acid gas from a natural gas feedstockto form a sweetened natural gas. The method includes flowing the naturalgas feedstock over a membrane, wherein the membrane includes acellulosic matrix and a polymer of intrinsic microporosity (PIM),wherein the PIM includes chains coupled by a heat-treating under vacuum.The portion of the acid gas is isolated in a permeate from the membrane.The sweetened natural gas is produced in a retentate from the membrane.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a drawing of the molecular structure of an example of apolymer of intrinsic microporosity, e.g., polymer of intrinsicmicroporosity-1 (PIM-1).

FIG. 1B is a space filling drawing of PIM-1, showing the pores of thestructure.

FIG. 2 is a schematic drawing of the coupling of PIM-1 chains undervacuum and high heat conditions.

FIG. 3 is a schematic drawing of a process for preparing membranes fromblends of cellulose acetate (CA) and PIM-1.

FIG. 4 is a schematic drawing of a method for the fabrication of aT-CA/PIM-1 membrane.

FIG. 5 is a simplified process flow diagram of a permeation apparatusused for measuring single gas and mixed gas permeation properties.

FIG. 6 is a plot of the tradeoff in membrane permeability andselectivity comparing membranes formed from neat CA, neat PIM, andCA/PIM tested under a single gas.

FIG. 7 is a plot comparing the effect of PIM-1 loading on CO₂/CH₄selectivity and CO₂ permeability in T-CA/PIM-1 membranes tested under asingle gas.

FIG. 8 is a plot of the tradeoff in membrane permeability andselectivity comparing membranes formed from a neat CA membrane andT-CA/PIM-1 membranes at different loadings of PIM-1 tested under abinary gas mixture.

FIG. 9 is a plot of the effect of PIM loading on CO₂/CH₄ membraneselectivity and CO₂ membrane permeability of T-CA/PIM-1 membranes incomparison to a neat CA membrane under a binary gas mixture.

FIGS. 10A and 10B are plots of the tradeoffs of membrane permeabilityversus membrane selectivity for CO₂/CH₄ vs. CO₂ and H₂S/CH₄ vs. H₂Scomparing neat CA thermally treated blends of CA/PIM at differentloading levels of PIM under sour mixed gas.

DETAILED DESCRIPTION

Polymeric membranes are thin semipermeable barriers that selectivelyseparate some gas compounds from others. These membranes do not operateas a filter, where small molecules are separated from larger onesthrough a medium with pores, rather they separate based on the rate ofdissolution and diffusion of compounds through the material of themembrane, termed the solution-diffusion model.

The use of polymeric membrane-based technology for gas separation hasgained industrial attention recently due to the potential for highenergy efficiency, small footprint, e.g., ease of adaptation intodifferent form factors, and low capital cost. There are many factorsaffecting the gas separation performance of a polymeric membrane. Highpermeation flux and gas-pair selectivity are regarded as two of the mostimportant criteria for the selection of a membrane for industrial use.However, there exists a trade-off between permeability and selectivity,as described herein.

Numerous polymeric membranes for gas separation have been developed overthe decades, but few are currently commercialized for use in sour gasseparation applications. Examples of polymeric materials used to formgas separation membranes include cellulose acetate (CA), polyimides(PI), and perfluoropolymers, such as polytetrafluoroethylene (PTFE),perfluorocycloalkene (PFCA), and the like. These polymeric materials aregenerally amorphous polymers that form glasses, for example, having aT_(g) of greater than about 100° C. CA is the most commercially usedglassy polymer for acid gas removal. For example, UOP LLC's Separex™ CAmembrane is used extensively for CO₂ removal from natural gas. However,for wider implementation, CA membranes may be improved in a number ofproperties, including permeability and selectivity and enhanced chemicaland thermal stability under the operating conditions that are typical ofgas fields, such as higher feed pressure and high acid gasconcentration.

To improve separation performance and stability, new membrane materials,including new polymeric materials and modifications of existingpolymeric materials have been studied. However, some barriers, such astrade-offs in selectivity versus permeability, may prevent thedeployment of these materials in industrial processes. Among variousapproaches, the use of polymer blends to form membranes has beenrecognized as one of the most promising routes as it combines theadvantages of different materials into a new compound with unique andsynergetic properties that are difficult to be obtained by syntheticmeans.

Embodiments described herein provide a method to produce a membrane froma polymer blend that has high permeability and selectivity. The matrixof the membrane is a cellulosic polymer, such as cellulose acetate (CA).A polymer of intrinsic microporosity (termed PIM herein) is a highlypermeable polymer that is blended into the cellulosic matrix. Membranesformed from these blends are thermally treated in a high temperaturevacuum oven to increase free volume (or pore) space by coupling PIMchains through a triazine ring formed during the heating. Theheat-treated membranes have been tested for sour gas separations, andexhibit significant improvements in the removal of CO₂ and H₂S from araw natural gas under a high-pressure gas feed when compared to neat CAmembranes.

FIG. 1A is a drawing of the molecular structure of an example of apolymer of intrinsic microporosity, e.g., polymer of intrinsicmicroporosity-1 (PIM-1). As described herein, PIMs are microporousmaterials with interconnected pores separated by less than 2 nm. PIM-1has demonstrated extremely high CO₂ permeability (>4000 Barrer) andrelatively low-to-moderate CO₂/CH₄ selectivity (about 11) under acid gasfeed testing conditions, due to its high free volume and high internalsurface area.

FIG. 1B is a space filling drawing of PIM-1, showing the pores 102 ofthe structure. PIMs have been compared to various inorganic microporousmaterials such as activated carbon and zeolites due to their largesurface area and highly rigid and contorted molecular structure thatprovide a large fractional free volume (FFV). This is shown for PIM-1 bythe pores 102 illustrated in the space filling drawing.

FIG. 2 is a schematic drawing 200 of the coupling of PIM-1 chains 100under vacuum and high heat conditions. Placing the PIM-1 under vacuumand high heat conditions can couple the chains through the formation ofa triazine ring 202 from functional groups (—CN moieties) attached tothe PIM-1, forming coupled PIM-1 structures 204.

FIG. 3 is a schematic drawing of a process 300 for preparing membranes302 and 304 from blends of CA and PIM-1. Like numbered items are asdescribed with respect to FIGS. 1 and 2. As described herein, a PIM,such as PIM-1 chains 100, is incorporated into a cellulosic matrix, suchas a CA matrix 306, from which a membrane is formed, such as theCA/PIM-1 membrane 302. The CA/PIM-1 membrane 302 has pores 308 formedfrom the PIM-1 chains 100, for example, as described with respect to thepores 102 of the PIM-1 chains 100 or due to pores formed as the PIM-1pushes apart the CA matrix 306. It may be noted that the term “pores” isused to refer to regions of free volume within the membranes 302 and304, and does not refer to accessible openings within or through themembranes 302 and 304. Although PIM-1 is shown in examples herein, thetechniques are not limited to the use of PIM-1. In other embodiments,other PIMs that include cyano moieties may be used, such as PIM-2,PIM-3, PIM-4, or PIM-5, among others.

The CA/PIM-1 membrane 302 is thermally treated in a high temperaturevacuum oven, for example, at about 100° C. to 250° C., or about 150° C.to about 180° C., or about 165° C., to form a thermally treated membranecomprising the coupled PIM-1 structures 204, termed a T-CA/PIM-1membrane 304, herein. The coupling of the PIM-1 chains 100 to form thecoupled PIM-1 structures 204 modifies the sizes of the pores 308 in theCA/PIM-1 membrane 302. In the T-CA/PIM-1 membrane 304, some pores 310are decreased in size and other pores 312 are increased in size incomparison to the pores 308 in the CA/PIM-1 membrane 302.

In tests described with respect to the examples, the T-CA/PIM-1 membrane304 showed improved permeability, such as to CO₂ and H₂S, andselectivity, for example, between CO₂/CH₄ and between H₂S/CH₄, comparedto a neat CA membrane. This is likely due to chain relaxation from thecoupling of the PIM-1 chains 100 creating a favorable morphology forenhancement of permeability and separation.

Examples Preparation of Membranes

FIG. 4 is a schematic drawing of a method 400 for the fabrication of aT-CA/PIM-1 membrane. The method begins with the preparation of a neat CAsolution. This is performed by adding dried CA powder 402 (Mw=50,000) totetrahydrofuran (THF). The neat CA solution formed is rolled tocompletely dissolve 404 it at room temperature. The concentration of theneat CA solution is between about 1 wt. % and about 10 wt. %, or betweenabout 4 wt. % and about 8 wt. %.

After the neat CA solution is prepared, a neat PIM-1 solution isprepared. To prepare the neat PIM-1 solution, a dried PIM-1 polymer 406was added to THF at different loadings, for example, from about 2.5 wt.% to about 95 wt. % of the amount of the CA used to form the neat CAsolution. The neat PIM-1 solution is rolled to completely dissolve 408at room temperature.

The neat CA solution and the neat PIM-1 solution were then mixed 410 toform a CA/PIM-1 solution. The CA/PIM-1 solution was then stirred for 60minutes to ensure homogeneity. The CA/PIM-1 solution was then leftunstirred for 30 minutes to release any air bubbles, and filtered toremove any solids.

After the CA/PIM-1 solution was prepared, a casting procedure 412 wasused to prepare the CA/PIM-1 membrane 414. For the casting procedure412, the CA/PIM-1 solution was poured into PTFE flat-bottomed Petridishes. The Petri dishes were covered to slow solvent evaporation andallowed to dry overnight at room temperature to prepare a dense film.The dense film was then dried in a vacuum oven at 80° C. for 48 hr toform the CA/PIM-1 membrane 414.

Once the CA/PIM-1 membrane 414 is formed, the T-CA/PIM-1 membrane 416 isfabricated. To fabricate the T-CA/PIM-1 membrane 416, the CA/PIM-1membrane 414 is heated 418 in a high temperature vacuum oven at atemperature in the range of about 150° C. to about 180° C. under avacuum pressure of less than about 10 mbar, with a heating rate of about10° C./min. The vacuum oven was held for a period of 72 hr at themaximum temperature. After the thermal treatment process was completed,the T-CA/PIM-1 membrane 416 was allowed to cool to room temperature inthe vacuum oven and stored in a drybox.

As described further with respect to the specific examples below, theT-CA/PIM-1 membrane 416 exhibited an increase over a neat CA membrane inmembrane permeability (CO₂ and H₂S) and selectivity (CO₂/CH₄ andH₂S/CH₄). The increases were maintained under different feed gases, forexample, pure, binary, and sour mixed gas, and testing conditions, forexample, feed pressures up to 800 psi.

Under single gas testing, for example, at a feed temperature of 25° C.and a feed pressure of 100 psi, T-CA/PIM-1 membranes with a PIM-1content of less than or about 40 wt. % show an increase over neat CAmembranes in CO₂/CH₄ selectivity of about 27% to about 33% and anincrease in CO₂ permeability of about 81% to about 139%. In one example,a T-CA/PIM-1 membrane had a CO₂/CH₄ single gas selectivity and a CO₂permeability of 44.28 and 1.07 Barrer, respectively, compared to a neatCA membrane, which had 33.21 and 4.64 Barrer, respectively.

When tested under a binary gas mixture testing (20% CO₂/80% CH₄) at 800psi, T-CA/PIM-1 blended membranes with addition of PIM-1 at less than orabout 40 wt. % showed significant improvements over neat CA membranes inCO₂/CH₄ mixed selectivity (41%-50%) and CO₂ permeability (140%-630%)under industrially-relevant testing conditions, e.g., feed pressure upto 800 psi. In one example, a T-CA/PIM-1 membrane had a CO₂/CH₄ mixedgas selectivity and a CO₂ permeability of 45.23 and 21.12 Barrer,respectively, compared to a neat CA membrane, which had 30.42 and 2.89Barrer, respectively.

The addition of the PIM-1 in the CA membrane matrix improved the sourgas separation performance under industrially relevant feed stream andtesting conditions, such as using a 3-component sour gas mixturecontaining 3% CO₂, 5% H₂S and 92% CH₄ and a feed pressure up to 800 psi.Under these conditions, T-CA/PIM-1 blended membranes show improvementsover neat CA membranes in membrane permeability (82%-328% and 95%-353%increase in CO₂ and H₂S, respectively) and selectivity (3% and 8%increase in CO₂/CH₄ and H₂S/CH₄ mixed gas selectivity, respectively).

Preparation of PIM-1 Polymer

The PIM-1 was prepared by dissolving equimolar amounts of purifiedmonomers, 2,3,5,6-tetra-fluoroterephthalonetrile (TFTPN, 44.063 mmol)and 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane(TTSBI, 44.063 mmol), in anhydrous DMAc/Toluene (90 mL/45 mL). Apolycondensation reaction to form the PIM-1 was catalyzed by K₂CO₃ undernitrogen atmosphere at 165° C. for 40 min. The polymer was precipitatedinto stirring methanol (1600 mL) overnight and washed with methanolseveral times. Finally, yellow powder was obtained after drying at 80°C. in vacuum oven overnight.

Preparation of Membranes Example Membranes Preparation of T-CA/PIMMembranes

CA/PIM-1 membranes were prepared by the techniques described withrespect to FIG. 4. In a typical membrane preparation procedure, a sampleof 0.8 g dried CA powder (MW=50,000) was dissolved in 10 mL THF(anhydrous, >99.9%, Sigma-Aldrich) in a sealed 25 mL glass vial and theCA solution was rolled to dissolve completely at room temperature.

Different amounts of PIM-1, for example, at a number of ratios between 1wt. % and 95 wt. % of the amount of CA used to form the CA solution,were each dissolved in 10 mL THF. The PIM-1 solutions formed were rolledto dissolve completely at room temperature.

Each of the PIM-1 solutions was mixed with a CA solution, and stirredovernight. The resulting CA/PIM-1 solutions included PIM-1concentrations of 2.5 wt. %, 5 wt. %, 10 wt. %, 20 wt. %, 30 wt. %, 40wt. %, 60 wt. %, 80 wt. %, 90 wt. %, and 95 wt. % of the amount of theCA.

Each of the CA/PIM-1 solutions were filtered through a 1 μm pore sizePTFE filter to remove impurities, and were then left unstirred for 30min to release air bubbles. The CA/PIM-1 solutions were then poured intoPTFE flat-bottomed Petri dishes to prepare dense films. The Petri disheswere covered to slow solvent evaporation and the CA/PIM-1 solutions wereallowed to dry at room temperature overnight to form the dense films.The Petri dishes with the dense films were then dried in a vacuum ovenat 80° C. for 48 hr. The resulting CA/PIM-1 membranes were allowed tocool to room temperature in the vacuum oven and stored in a drybox forfurther studies. The CA/PIM-1 membranes obtained had thicknesses in therange of 10 to 90 μm, as determined by scanning electron microscopy(SEM) images of membrane cross sections, taken on a JEOL 7100F SEM.

Each of the CA/PIM-1 membranes were given a further thermal treatment ina vacuum oven to prepare the T-CA/PIM-1 membranes. After a vacuum ofless than about 10 mbar was achieved, the vacuum oven temperature wasraised to between 150° C. and 250° C. at a rate of 10° C./min and heldfor a period of 72 hr. After the thermal treatment process, themembranes were allowed to cool to room temperature in the vacuum ovenand stored in a drybox for further studies. The T-CA/PIM-1 membranesobtained had thicknesses in the range of 10 to 90 μm, as determined bySEM.

Comparison Membranes

Preparation of Cellulose Acetate Membranes (Neat CA Membranes)

The neat cellulose acetate (CA) membrane was prepared using a solutioncasting technique, similar to that described with respect to FIG. 4. Asample of 0.8 g of dried cellulose acetate powder (MW=50,000) was addedinto 20 mL of THF. The neat CA solution was rolled to dissolvecompletely at room temperature. The neat CA solution was then filteredthrough a 1 μm pore size PTFE filter to remove impurities, and pouredinto a PTFE flat-bottomed Petri dish to prepare a dense film. The CApolymer solution was covered to slow solvent evaporation, and dried atroom temperature for 48 hr. After 48 hours at room temperature, thePetri dish was placed in a vacuum oven and dried at 80° C. for 48 hr andthen at 110° C. for another 48 hr. The membrane was cooled naturally inthe vacuum oven to room temperature and stored in a drybox for furtherstudies. For comparison, multiple neat CA membranes were prepared usingthis procedure. The neat CA membranes obtained had an average thicknessof 10 to 90 μm, as determined by SEM, and were easily peeled off thePetri dishes for permeation testing.

Preparation of Polymer of Intrinsic Microporosity Membranes (Neat PIM-1Membrane)

The neat polymer of intrinsic microporosity (PIM-1) membrane wasprepared using the solution casting technique, similar to that describedwith respect to FIG. 4. A sample of 0.4 g of PIM-1 was added into 10 mLTHF. The PIM-1 solution was rolled to dissolve completely at roomtemperature. The PIM-1 solution was then filtered with a 1 μm pore sizePTFE filter to remove impurities, and poured into a PTFE flat-bottomedPetri dish to prepare a dense film. The Petri dish was covered to slowsolvent evaporation, and dried at room temperature for 48 hr. After 48hours at room temperature, the Petri dish was placed in a vacuum ovenand dried at 80° C. for 48 hr. The membrane was allowed to cool to roomtemperature in the vacuum oven and stored in a drybox for furtherstudies. For comparison, multiple PIM-1 membranes were prepared usingthis technique. The PIM-1 membranes had an average thickness of 40 to110 μm, as determined by SEM, and were easily peeled off the Petridishes for permeation testing.

Membrane Permeation Testing

FIG. 5 is a simplified process flow diagram of a permeation apparatus500 used for measuring single gas and mixed gas permeation properties.The permeation apparatus 500 is a custom-built unit. A feed gas tank 502holds the gas or gas mixtures used for the tests. Multiple tanks coupledby a gas manifold may be used if sequential gas tests are desirable. Aconstant temperature enclosure or oven 504 holds the test apparatus,including a constant volume reservoir 506 and the permeation cell 508.The constant volume reservoir 506 ensures that the volume of gases onthe membranes under test will remain constant during test parameterchanges. The permeation cell 508 is a stainless-steel permeation cellusing 47 mm disc filters, purchased from EMD Millipore. A vacuum pump510 is used to clear the lines of undesirable gases, such as oxygen,nitrogen, and other atmospheric gases, in preparation for the testing. Agas chromatograph 512 is used to detect and quantify the amounts andratios of gases in the feed to the permeation cell 510 and in the outletfrom the permeation cell 508.

Permeation Test Procedures

The gas permeation tests were performed in triplicate using aconstant-volume, variable-pressure technique. In this technique, atesting gas (upstream side or feed side) with desired pressure is fed tothe permeation cell 508. In the variable-pressure method, the gaspermeates through a membrane film into a closed, constant-volume chamber(downstream side, or permeate side) that is evaluated. The downstreampressure rise in the chamber is recorded as a function of time.

For testing, an epoxy masked membrane sample of 5-20 mm in diameter wasinserted and sealed in the permeation cell 508. The permeation apparatus500 was then evacuated for 1 hour before each test, using the vacuumpump 510. Pure gas permeability coefficients were measured at thetemperature range of 20° C. to 50° C. and feed pressure range of 25 to700 psi in the order of CH₄ followed by CO₂ to avoid swelling.

In addition to single gas tests, binary gas mixtures were used forpermeation tests, including a CO₂/CH₄ mixture (20/80 vol/vol). Two sourgas feeds were also used for tests. The first is termed a 5% sour gasmixture and included three components, 5 vol. % H₂S, 3 vol. % CO₂, and92 vol. % CH₄. The second sour gas feed, referred to herein as a 20%sour gas mixture, included five components, 10 vol. % CO₂, 20 vol. %H₂S, 10 vol. % N₂, 3 vol. % C₂H₆, and 57 vol. % CH₄.

Steady-state permeation was verified using the time-lag method, where 10times the diffusion time lag was taken as the effective steady state. Asused herein, the time-lag method is commonly used to characterizemembrane permeation properties. The intercept on the time axis of theplot of pressure rise versus time is defined as the time lag, t. Theupstream (feed) pressure and the downstream (permeate) pressure weremeasured using Baraton absolute capacitance transducers (MKSInstruments) and recorded using LabVIEW software. The permeate pressurewas maintained below 100 torr using a second vacuum pump 814. Mixed gaspermeation was performed at 20° C. and feed pressure range of 200 psi to800 psi with binary gas mixture and sour gas mixtures. A retentatestream was added for mixed gas tests and adjusted to 100 times thepermeate flow rate to maintain a less than 1% stage cut. As used herein,the stage cut is a ratio of permeate flow to feed flow, and is definedas the fraction of feed gas that permeates the membrane, and is ameasure of the degree of separation. The permeate gas was collected andthen injected into the gas chromatograph 512, which was a Shimadzu gaschromatograph (GC-2014), to measure permeate composition. Permeateinjections were performed at 95 torr. An Isco model 1000D syringe pump(TeledyneIsco) was used to control the feed pressure.

The permeability coefficients of gas i, P_(i), were calculated accordingto Equation 1. In equation 1, dp_(i)/dt is the slope of the steady statepressure rise in the downstream, Vis the downstream volume, R is theideal gas constant, Tis the temperature of the downstream, L is themembrane thickness (as determined by SEM), A is the membrane surfacearea (estimated using ImageJ image processing software), and Δf_(i) isthe partial fugacity difference across the membrane calculated using thePeng-Robinson equation. Selectivity, α_(i/f), was calculated as theratio of permeability coefficients as expressed in Equation 2.

$\begin{matrix}{P_{i} = {\frac{dP_{i}}{d_{t}}\frac{VL}{{RTA}\;\Delta\; f_{i}}}} & (1) \\{\alpha_{i/j} = \frac{P_{i}}{P_{j}}} & (2)\end{matrix}$

Membrane Pure Gas Permeation Properties

FIG. 6 is a plot of the tradeoff in membrane permeability andselectivity comparing membranes formed from neat CA, neat PIM, andCA/PIM tested under a single gas. Over the last several decades,membrane researchers have developed hundreds of polymeric membranes forgas separation. Studies have determined that there exists a trade-off inbehavior between permeability and selectivity, which has beenquantitated by studying numerous polymeric membranes. This is indicatedin FIG. 6 by the line labeled “1991 trade-off” As polymeric membraneshave continued to improve in permeability and selectivity, the line hasshifted to a higher level, as indicated by the line labeled “2008trade-off.” These trade-off lines are referred to as the Robeson upperbound lines.

The results shown are for the membrane permeability-selectivitytrade-off (CO₂/CH₄ vs. CO₂) comparison of neat CA membrane 602 (opensquare), neat PIM-1 membrane 604 (open circle), and T-CA/PIM-1 membranes606 (solid squares). The PIM-1 loadings for the T-CA/PIM-1 membranes 606as a weight percent (wt. %) of the CA matrix are (from left to right)2.5 wt. %, 5 wt. %, 10 wt. %, 20 wt. %, 30 wt. %, 40 wt. %, 80 wt. %, 90wt. %, and 95 wt. %. The tests were performed in pure gas at 25° C. and100 psi.

The ideal transport properties are measured by the pure gas permeationand provide preliminary material observations and comparisons. Theresults show that the CO₂/CH₄ gas separation performance is within theRobeson upper bound lines (1991 and 2008). Some of the T-CA/PIM-1membranes 606, for example, at a loading of PIM-1 or at 80 wt. % orgreater, have separation performance that is above the 1991 upper bound.However, these membranes have lower CO₂/CH₄ selectivity and are notsuitable for actual gas separation.

The incorporation of lower amounts of PIM-1 into the T-CA/PIM-1 membrane606, for example, 2.5-40 wt. % in the CA matrix, not only leads to asignificant increase in gas permeability, but also enhanced selectivity,which makes the membranes suitable for CO₂/CH₄ separation. In oneexample (Table 1), T-CA/PIM-1 membranes at CA/PIM-1 blend ratios of95/5, 90/10, and 80/20 had CO₂/CH₄ selectivities of 42.18, 43.85 and44.28, respectively, which are a 27% to 32% increase over the neat CAmembrane 602, which had a selectivity of 33.21.

Further, the T-CA/PIM-1 blended membranes 606 also exhibited significantenhancement in permeability (81%, 88% and 139% increase in CO₂permeability for T-CA/PIM-1 (95/5), T-CA/PIM-1 (90/10) and T-CA/PIM-1(80/20), respectively), compared to the neat CA membrane 602. Bycomparison, the addition of PIM-1 into other glassy polymer membranes,such as Matrimid®, Ultem®, Torlon®, PAFEK®, P84®, TB®, without thermaltreatment in vacuum conditions, leads to an increase in CO₂permeability, but a decrease in CO₂/CH₄ selectivity.

FIG. 7 is a plot comparing the effect of PIM-1 loading on CO₂/CH₄selectivity 702 and CO₂ permeability 704 in T-CA/PIM-1 membranes testedunder a single gas in comparison to a neat CA membrane. In this test,the PIM-1 loadings as a weight percent of the matrix are 2.5 wt. %, 5wt. %, 10 wt. %, 20 wt. %, 30 wt. %, 40 wt. %, 80 wt. %, 90 wt. %, and95%. The tests were performed under a binary gas mixture (20% CO₂/80%CH₄) at 25° C. and 800 psi.

As shown in FIG. 7, the loading of PIM-1 has a significant influence onthe membrane separation performance. Under pure gas testing, theT-CA/PIM-1 membranes show a gradual increase in CO₂ permeability 704with increase of PIM-1 loading below a loading of about 40 wt. %, but afast increase at higher PIM loading, e.g., greater than about 40 wt. %.On the other hand, the selectivity 702 for CO₂/CH₄ increases up to aPIM-1 loading of about 20 wt. %, then decreases at PIM-1 loadings abovethat level. These results indicate that the membrane separationperformance can be adjusted by the PIM-1 loading in a CA matrix.

TABLE 1 Single gas permeation results for neat CA membrane, thermallytreated CA/PIM-1 blended membranes (T-CA/PIM-1), and neat PIM-1 membranefor CO₂/CH₄ separation P_(CO2) P_(CO2) P_(CH4) α_(CO2/CH4) Membranes(Barrer) increased (Barrer) α_(CO2/CH4) increased Neat CA membrane 4.64— 0.14 33.21 — T-CA/PIM-1 membrane 8.38  81% 0.20 42.18 27% (95/5)T-CA/PIM-1 membrane 8.74  88% 0.20 43.85 32% (90/10) T-CA/PIM-1 membrane11.07 139% 0.25 44.28 33% (80/20) T-CA/PIM-1 membrane 11.57 149% 0.2532.96 −1% (60/40) Neat PIM-1 membrane 3906.24 — 35.41 11.03 — Tested at25° C. and feed pressure of 100 psi

Membrane Permeation Properties for Binary Gas Mixture

FIG. 8 is a plot of the tradeoff in membrane permeability andselectivity comparing membranes formed from a neat CA membrane 802 andT-CA/PIM-1 membranes 804 at different loadings of PIM-1 tested under abinary gas mixture. The binary gas mixture contained 20% CO₂ and 80% CH₄to allow a determination as to whether interactions between one of thegas species and the membrane influences the permeation of the other gasspecies through the membrane.

FIG. 8 shows the separation performance against the Roberson upperbounds for CO₂/CH₄ separation at the feed pressure of 800 psi. Similarto the pure gas test results shown in FIG. 6, the binary gas mixtureseparation performance is within the upper bound line as the PIM-1loading increases in the CA matrix.

For CO₂/CH₄ separation, the addition of 2.5 wt. %-40 wt. % of PIM-1increases the CO₂/CH₄ mixed gas selectivity and CO₂ permeability forT-CA/PIM-1 membranes 804. In one example (Table 3), T-CA/PIM-1 membranes804 at CA/PIM-1 ratios of 97.5/2.5, 95/5, 90/10, and 80/20 had CO₂/CH₄mixed gas selectivities of 42.80, 42.82, 45.64, and 45.23, respectively,at a feed pressure of 800 psi. These values are a 41%, 41%, 50%, and 49%increase, respectively, over a neat CA membrane 802, which had a CO₂/CH₄mixed gas selectivity of 30.42.

A number of T-CA/PIM-1 membranes also exhibited an increase inpermeability over the neat CA membrane 802. For example, for T-CA/PIM-1at CA/PIM-1 ratios of 97.5/2.5, 95/5, 90/10, and 80/20 increases in CO₂permeability were seen over a neat CA membrane of 140%, 233%, 313% and631% increase, respectively. Thus, T-CA/PIM-1 membranes demonstratebetter overall separation performance under mixed gas testingconditions.

FIG. 9 is a plot of the effect of PIM loading on CO₂/CH₄ membraneselectivity 902 and CO₂ membrane permeability 904 of T-CA/PIM-1membranes under a binary gas mixture. The tests were run with a binarygas mixture of 20% CO₂/80% CH₄, at 25° C. and 800 psi.

However, the overall mixed gas selectivity for all of the testedmembranes are comparable to their ideal selectivity, but at higher CO₂permeability. For example, a T-CA/PIM-1 membrane with a loading of 80/20CA/PIM-1, has a mixed gas CO₂ permeability of 21.12 Barrer, which is 91%higher than its pure gas CO₂ permeability of 11.07 Barrer. This increasein permeability may be due to the better sorption interactions of CO₂ ina mixed gas and the higher affinity of PIM-1 towards CO₂. Similar trendswere observed under binary gas mixture tests for T-CA/PIM-1 membraneswith a loading of 10%-20%, which have higher CO₂/CH₄ mixed gasselectivity in the range of PIM-1 loading of 10%-20%.

TABLE 2 Binary gas mixture (20% CO₂/80% CH₄) permeation results of neatCA membrane and T-CA/PIM-1 membranes for CO₂/CH₄ separation P_(CO2)P_(CO2) P_(CH4) α_(CO2/CH4) Membranes (Barrer) increased (Barrer)α_(CO2/CH4) increased Neat CA membrane 2.89 — 0.095 30.42 — T-CA/PIM-1membrane 6.93 140% 0.163 42.80 41% (97.5/2.5) T-CA/PIM-1 membrane (95/5)9.64 233% 0.225 42.82 41% T-CA/PIM-1 membrane (90/10) 11.95 313% 0.26245.64 50% T-CA/PIM-1 membrane (80/20) 21.12 631% 0.467 45.23 49%T-CA/PIM-1 membrane (20/80) 1674.10 57827%  187.9 8.91 −71%   T-CA/PIM-1membrane (5/95) 3029.89 104741%   299.97 11.03 −67%   Tested at 25° C.and feed pressure of 800 psi

Membrane Sour Mixed Gas Permeation Properties

FIGS. 10A and 10B are plots of the tradeoffs of membrane permeabilityversus membrane selectivity for CO₂/CH₄ vs. CO₂ and H₂S/CH₄ vs. H₂Scomparing neat CA thermally treated blends of CA/PIM at differentloading levels of PIM. The testing was performed under a sour gas feed(3% CO₂/5% H₂S/92% CH₄) at 25° C. and 800 psi.

FIG. 10A shows the CO₂/CH₄ vs. CO₂ permeability-selectivity trade-offcomparison of neat CA membrane 1002 (open square), neat PIM-1 membrane1004 (open circle), and T-CA/PIM-1 membranes 1006 (solid blue triangles,from left to right: PIM-1 loadings are 2.5 wt. %, 5 wt. %, 10 wt. %, 20wt. %, and 80 wt. %). As seen in other tests, the permeation propertiesof the T-CA/PIM-1 membranes 1006 lie below the upper bound line whenPIM-1 loading increases in the CA matrix. FIG. 10B shows the H₂S/CH₄ vs.H₂S permeability-selectivity trade-off comparison of the same materials.

However, the T-CA/PIM-1 membranes 1006 show significant improvement overa neat CA membrane in permeability (80%˜328% increase in CO₂permeability and 77%˜353% increase in H₂S permeabilities), andcomparable CO₂/CH₄ and H₂S/CH₄ mixed gas selectivities with the additionof amounts of PIM-1 of less than about 20 wt. %. In one example (Table4), a T-CA/PIM-1 membrane with a PIM-1 loading of 5 wt. % had CO₂/CH₄and H₂S/CH₄ mixed gas selectivities of 36.66 and 43.11, CO₂ and H₂Spermeabilities of 7.50 Barrer and 8.90 Barrer, respectively, compared toneat CA membrane CO₂/CH₄ and H₂S/CH₄ mixed gas selectivities of 35.74and 39.76, and CO₂ and H₂S permeabilities of 4.11 Barrer and 4.57Barrer, respectively.

TABLE 3 Sour mixed gas permeation properties for neat CA membrane, neatPIM-1 membrane and thermally treated CA/PIM-1 blended membranes(T-CA/PIM) P_(CO2) P_(CO2) P_(H2S) P_(H2S) α_(CO2/CH4) α_(H2S/CH4)Membranes (Barrer) increased (Barrer) increased α_(CO2/CH4) increasedα_(H2S/CH4) increased Neat CA 4.11 — 4.57 — 35.74 — 39.76 — T-CA/PIM-7.50  82% 8.90  95% 36.66  3% 43.11  8% 1(95/5) T-CA/PIM- 7.40  80% 8.10 77% 36.79  3% 40.14  1% 1(90/10) T-CA/PIM- 17.60 328% 20.70 353% 33.60−6% 38.91 −2% 1(80/20) Neat PIM- 478.90 — 250.00 — 15.72 — 31.82 — 1Sour mixed gas composition: 3% CO₂/5% H₂S/92% CH₄ Feed temperature: 25°C.; feed pressure: 800 psi

An embodiment described in examples herein provides a gas separationmembrane. The gas separation membrane includes a cellulosic matrix and apolymer of intrinsic microporosity (PIM). The PIM includes chainscoupled by a heat-treating under vacuum.

In an aspect, the cellulosic matrix includes cellulose acetate (CA). Inan aspect, the cellulosic matrix includes cellulose acetate butyrate(CAB).

In an aspect, the PIM includes PIM-1. In an aspect, the PIM includestriazine rings coupling PIM chains.

In an aspect, the gas separation membrane includes larger free volumespaces and smaller free volume spaces formed by the heat-treating undervacuum. In an aspect, the gas separation membrane has a CO₂ permeabilitythat is at least 50% higher than the CO₂ permeability of a neat CAmembrane. In an aspect, the gas separation membrane has a CO₂/CH₄ mixedgas selectivity that is within +/−10% of a neat CA membrane. In anaspect, the gas separation membrane has an H₂S permeability that is atleast 50% higher than the H₂S permeability of a neat CA membrane. In anaspect, the gas separation membrane has an H₂S/CH₄ mixed gas selectivitythat is within +/−10% of a neat CA membrane.

In an aspect, the gas separation membrane includes between about 2.5 wt.% PIM and about 40 wt. % PIM. In an aspect, the gas separation membraneincludes between about 5 wt. % PIM and about 20 wt. % PIM.

Another embodiment described in examples herein provides a method forforming a gas separation membrane. The method includes forming acellulosic polymer solution, forming a polymer of intrinsicmicroporosity (PIM) solution, and blending the polymer solution of thecellulosic polymer with the polymer solution of the PIM to form a mixedpolymer solution. A dense film is formed from the mixed polymersolution. The dense film is dried. The dense film is heat-treated underheat and vacuum to couple the PIM chains.

In an aspect, the method includes dissolving a cellulosic polymer intetrahydrofuran (THF) to form the cellulosic polymer solution. In anaspect, the method includes forming the cellulosic polymer solution bydissolving cellulose acetate in THF.

In an aspect, the method includes dissolving the PIM in THF. In anaspect, the method includes forming the PIM polymer solution bydissolving PIM-1 in THF. In an aspect, the method includes dissolving anamount of PIM that is in a weight ratio to the cellulosic polymer ofbetween about 2.5 wt. % and about 40 wt. %. In an aspect, the methodincludes dissolving an amount of PIM that is in a weight ratio to thecellulosic polymer of between about 5 wt. % and about 20 wt. %.

In an aspect, the method includes forming triazine rings during the heattreatment under vacuum, wherein the triazine rings couple the PIMchains.

Another embodiment described in examples herein provides a method forremoving at least a portion of an acid gas from a natural gas feedstockto form a sweetened natural gas. The method includes flowing the naturalgas feedstock over a membrane, wherein the membrane includes acellulosic matrix and a polymer of intrinsic microporosity (PIM),wherein the PIM includes chains coupled by a heat-treating under vacuum.The portion of the acid gas is isolated in a permeate from the membrane.The sweetened natural gas is produced in a retentate from the membrane.

In an aspect, the cellulosic matrix is formed from cellulose acetate(CA). In an aspect, the PIM is formed from PIM-1. In an aspect, the PIMincludes triazine rings formed by the heat-treating under vacuum. In anaspect, larger free volume spaces and smaller free volume spaces areformed by the heat-treating under vacuum.

Other implementations are also within the scope of the following claims.

What is claimed is:
 1. A gas separation membrane, comprising: acellulosic matrix; and a polymer of intrinsic microporosity (PIM),wherein the PIM comprises chains coupled by a heat-treating undervacuum.
 2. The gas separation membrane of claim 1, wherein thecellulosic matrix comprises cellulose acetate (CA).
 3. The gasseparation membrane of claim 1, wherein the cellulosic matrix comprisescellulose acetate butyrate (CAB).
 4. The gas separation membrane ofclaim 1, wherein the PIM comprises PIM-1.
 5. The gas separation membraneof claim 1, wherein the PIM comprises triazine rings coupling PIMchains.
 6. The gas separation membrane of claim 1, comprising largerfree volume spaces and smaller free volume spaces formed by theheat-treating under vacuum.
 7. The gas separation membrane of claim 1,comprising a CO₂ permeability that is at least 50% higher than the CO₂permeability of a neat CA membrane.
 8. The gas separation membrane ofclaim 1, comprising a CO₂/CH₄ mixed gas selectivity that is within+/−10% of a neat CA membrane.
 9. The gas separation membrane of claim 1,comprising an H₂S permeability that is at least 50% higher than the H₂Spermeability of a neat CA membrane.
 10. The gas separation membrane ofclaim 1, comprising an H₂S/CH₄ mixed gas selectivity that is within+/−10% of a neat CA membrane.
 11. The gas separation membrane of claim1, comprising between about 2.5 wt. % PIM and about 40 wt. % PIM of theamount of the CA in the membrane.
 12. The gas separation membrane ofclaim 1, comprising between about 5 wt. % PIM and about 20 wt. % PIM ofthe amount of the CA in the membrane.
 13. A method for forming a gasseparation membrane, comprising: forming a cellulosic polymer solution;forming a polymer of intrinsic microporosity (PIM) solution; blendingthe polymer solution of the cellulosic polymer with the polymer solutionof the PIM to form a mixed polymer solution; forming a dense film fromthe mixed polymer solution; drying the dense film; and heat-treating thedense film under heat and vacuum to couple the PIM chains.
 14. Themethod of claim 13, comprising dissolving a cellulosic polymer intetrahydrofuran (THF) to form the cellulosic polymer solution.
 15. Themethod of claim 13, comprising forming the cellulosic polymer solutionby dissolving cellulose acetate in THF.
 16. The method of claim 13,comprising dissolving the PIM in THF.
 17. The method of claim 13,comprising forming the PIM polymer solution by dissolving PIM-1 in THF.18. The method of claim 13, comprising dissolving an amount of the PIMthat is in a weight ratio to the cellulosic polymer of between about 2.5wt. % and about 40 wt. %.
 19. The method of claim 13, comprisingdissolving an amount of the PIM that is in a weight ratio to thecellulosic polymer of between about 5 wt. % and about 20 wt. %.
 20. Themethod of claim 13, comprising forming triazine rings during the heattreatment under vacuum, wherein the triazine rings couple the PIMchains.
 21. A method for removing an acid gas from a natural gasfeedstock to form a sweetened natural gas, comprising: flowing thenatural gas feedstock over a membrane, wherein the membrane comprises: acellulosic matrix; and a polymer of intrinsic microporosity (PIM),wherein the PIM comprises chains coupled by a heat-treating undervacuum; and isolating the acid gas in a permeate from the membrane; andproducing the sweetened natural gas in a retentate from the membrane.22. The method of claim 21, comprising forming the cellulosic matrixfrom cellulose acetate (CA).
 23. The method of claim 21, comprisingforming the PIM from PIM-1.
 24. The method of claim 21, wherein the PIMcomprises triazine rings formed by the heat-treating under vacuum. 25.The method of claim 21, comprising forming larger free volume spaces andsmaller free volume spaces by the heat-treating under vacuum.